* WP_Query is a built in wordpress class allows you to query registered posts such as posts,
* pages and custom post types (e.g portfolio) check custom_portfolio at function.php
* have_posts is a method from WP_Query class to check if there are rows (or posts) at the requested query.
* the_post will make variable accessable inside the while loop (e.g the post ID.)
* get_the_post_thumbnail_url wordpress function that accepts 2 parameters ID and size id (check functions.php line 8) and returns the featured image url of a post.
* the_permalink wordpress function to return post url